The present invention relates to optical communication systems and more particularly to amplification in optical communication systems.
The enormous growth in telecommunication traffic is driving the development of technology to greatly expand the available bandwidth of backbone networks. In particular, there is a great impetus towards increasing the capacity of optical communication links and reducing the costs of implementing capacity-increasing technologies.
One fruitful avenue for increasing the capacity of optical communication links has been combining optical signals with multiple wavelengths onto the same fiber, a practice referred to as wavelength division multiplexing (WDM). Key to the development of WDM systems is the development of optical amplifier technology capable of boosting optical signals in a way that is transparent to data rate and format and that can provide effective amplification across a very large bandwidth.
Continuing development of WDM systems, including the advent of so-called DWDM (dense WDM) systems, is increasing the challenges inherent in optical amplifier design. Envisioned future DWDM systems will carry hundreds of very closely spaced optical signals. Spacings of less than 25 GHz are anticipated. Amplifying such a DWDM signal is problematic for a number of reasons. For example, to overcome the nonlinearity distortion inherent in such close channel spacings, current Raman amplifier technology requires very high laser pump power to achieve acceptable gain in very short fibers or in fibers with very low nonlinear coefficients (low material nonlinear coefficient and large effective area) and consequently very low Raman gain coefficient. The additional pump power comes at a high cost.
The very high cost of optical amplifiers capable of amplifying large numbers of closely spaced optical signals creates economic difficulties for service providers planning to install high capacity optical links. When a high capacity DWDM link is deployed, only a small portion of the available bandwidth is used initially with the rest being reserved for future expansion. A large up-front investment must therefore be made even though effective commercial exploitation of the new equipment remains quite far in the future.
What is needed are systems and methods for optical amplification that provide a very high system capacity but take into account relevant economic constraints.
By virtue of one embodiment of the present invention, a modular interleaved Raman amplifier structure is exploited to reap the advantages provided by the high Raman gain coefficient and small effective area of highly nonlinear fibers without incurring penalties caused by nonlinear effects and double-Rayleigh backscattering noise. Very tight WDM channel spacings may be accommodated. The amplifier structure may be implemented at very low initial cost and with high reliability, scalability, and modularity.
A first aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals, each of the Raman optical amplifiers including nonlinear fiber that is pumped with optical pump energy. A Raman gain coefficient of the nonlinear fiber is greater than or equal to approximately 3xc3x9710xe2x88x923Wxe2x88x921mxe2x88x921.
A second aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals, and a plurality of semiconductor lasers to provide optical pump energy to the plurality of Raman optical amplifiers.
A third aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals, each of the Raman optical amplifiers including nonlinear fiber that is pumped with optical pump energy. The nonlinear fiber has an length of less than or equal to approximately 5 km in each of the Raman optical amplifiers.
A fourth aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals, each of the Raman optical amplifiers including nonlinear fiber that is pumped with optical pump energy. At least one of the plurality of Raman optical amplifiers is divided into at least two isolated stages.